$addFields (aggregation)

The $addFields stage in the aggregation pipeline is used to add new fields to documents. It can also be used to reset the values of existing fields. This stage is particularly useful when you need to create new fields based on existing data or modify existing fields within your documents.

The basic syntax for the $addFields stage is as follows:

{
  $addFields: {
    <newField1>: <expression1>,
    <newField2>: <expression2>,
    ...
  }
}

Suppose we have a collection named stores with documents similar to the provided JSON structure. To add a new field totalDiscountEvents that counts the number of promotion events, we can use the following aggregation pipeline:

db.stores.aggregate([
  {
    $addFields: {
      totalDiscountEvents: { $size: "$store.promotionEvents" }
    }
  }
])

Sample output

[
  {
    "_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
    "store": {
      "name": "Downtown Store",
      "promotionEvents": ["Summer Sale", "Black Friday", "Holiday Deals"]
    },
    "totalDiscountEvents": 3
  }
]

If we want to add a field totalStaffCount that sums up the full-time and part-time staff, we can use:

db.stores.aggregate([
  {
    $addFields: {
      totalStaffCount: {
        $add: ["$store.staff.totalStaff.fullTime", "$store.staff.totalStaff.partTime"]
      }
    }
  }
])

Sample output

[
  {
    "_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
    "store": {
      "name": "Downtown Store",
      "staff": {
        "totalStaff": {
          "fullTime": 12,
          "partTime": 8
        }
      }
    },
    "totalStaffCount": 20
  }
]

To add a nested field location.coordinates that combines latitude and longitude into an array, use:

db.stores.aggregate([
  {
    $addFields: {
      "store.location.coordinates": ["$store.location.lat", "$store.location.lon"]
    }
  }
])

Sample output

[
  {
    "_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
    "store": {
      "name": "Downtown Store",
      "location": {
        "lat": 47.6097,
        "lon": -122.3331,
        "coordinates": [47.6097, -122.3331]
      }
    }
  }
]

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